Optical technique for detecting buried defects in opaque films

ABSTRACT

A local area of a sample is focally heated to produce a transient physical deformation. The surface of the structure is optically monitored while the heated area cools to a baseline temperature by illuminating the heated region with one or more probe beams from time to time and detecting returning light. In some embodiments heat dissipation within the structure is correlated with change in optical reflectivity over time. In other embodiments, surface deformation of the structure is correlated with changes in light scattering from the surface. Following application of a pump pulse and no more than 3 probe pulses, a time varying returning light signal is compared with a corresponding returning light signal from a reference. An anomaly in the sample is indicated by a deviation between the two signals. First-degree exponential decay curves may be constructed from the signals, and their decay constants compared.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Provisional ApplicationNo. 60/378,729, titled Optical technique for detecting buried defects inopaque films” filed May 7, 2002 and claims the benefit of United Statesprovisional patent application No. 60/378,400 filed May 6, 2002 titled“High speed laser inspection system”.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to fabrication of semiconductor devices.More particularly, this invention relates to the detection of subsurfacedefects in non-homogeneous structures such as multilayered integratedcircuits.

[0004] 2. Description of the Related Art

[0005] Semiconductor structures are inspected prior to, during, andafter patterning procedures. Patterned metal films used in integratedcircuit devices are often created using a damascene technique, in whicha pattern is etched in an insulating dielectric layer, and subsequentlyfilled using any of several standard deposition techniques, e.g.,chemical vapor deposition (CVD), physical vapor deposition (PVD), orelectro-copper plating (ECP). In the course of this process defects maybe created inside or under the metal, such as voids, delamination,underfill or underetch of the dielectric, and other interface-relateddefects.

[0006] Generally, these defects are not directly accessible usingoptical inspection techniques due to opacity of the surface layer. Tosome degree, they may be detected using voltage-contrast scanningelectron microscopy (SEM), or electron beam inspection (EBI). Althoughburied defects can sometimes be seen using SEM, that technique is betteradapted for evaluation of particular positions on a substrate or wafer,rather than for scanning. EBI is an inspection technique employing ascanning electron beam, which can scan significant portions of an entirewafer to automatically find defects. It can in principle see whatever aSEM tool sees. However, EBI tools are generally too slow and expensivefor the production floor. They are typically used in the research anddevelopment stage of product development.

[0007] It is proposed in U.S. Pat. No. 4,710,030 to employ a pump beamof short, non-destructive laser pulses (0.01-100 ps duration) to inducea thermo-elastic deformation, or stress waves, in a structure beingtested, and to monitor the transient response of the structure using alow-power laser probe beam that is directed to the area of thedeformation. By analyzing the intensity of the returning probe beam,information regarding defects and other characteristics of the structurecan be inferred.

[0008] Besides reflections of short-pulse-induced stress waves, voidsand interface defects are known to produce other physical effects inresponse to a pump beam, such as changes in acoustic dispersionproperties, and reduced heat dissipation. These effects are discussed inthe document Pico-second Ultrasonics, Grahn et al., IEEE Journal ofQuantum Electronics, Vol. 25 No. 12, pp. 2562-2568 (December 1989).

[0009] U.S. Pat. No. 5,633,711 discloses another example of monitoringthe transient response to an excitation laser pulse that impinges on andlocally heats a structure. In this disclosure, besides the intensity ofthe probe beam, phenomena such as acoustic oscillations and polarizationdisturbances are taken into account.

[0010] A disadvantage of the techniques disclosed in the above-notedpatents is a low signal-to-noise ratio (SNR) in the detection signals.The stress wave produced by the pump beam is associated with very smallchanges in reflectivity (expressed as a percent of the light falling onthe surface). Values from 1×10⁻⁶ to 1×10⁻⁴ are typical. It has thus beennecessary to compensate for the poor SNR by repeating the detectionsequence over a relatively long period, for example, a second for eachdetection spot. Many repetitions of the detection sequence are generallyrequired to obtain meaningful data. Furthermore, the frequency ofrepetition is itself limited by the need to perform mechanicaladjustments in the detection unit between performance cycles. Thus, thetime needed to evaluate a structure becomes impracticably long forfull-wafer inspection purposes.

[0011] U.S. Pat. No. 6,320,666 discloses an intensity modulated pumplaser beam, which is focused onto a sample so as to excite the sampleperiodically. Periodic heating by the pump beam creates a time varyingdeformation in the sample surface. A probe laser beam, obtained from asecond laser, is focused onto the sample within the periodically heatedarea. The pump and probe beam are spaced apart, and the probe beam issaid to undergo periodic angular deviations at the frequency of themodulated heating. A photodetector is provided for monitoring thereflected power of the probe beam and generating an output signalresponsively thereto. The output signal is filtered and processed toprovide a measure of the modulated optical reflectivity of the sample. Asteering apparatus is provided for adjusting the relative position ofpump and probe beam spots on the sample surface. The steering apparatusis used to move the beam spots from an overlapping, aligned position, toa position of separation of up to about 10 microns. Measurements can betaken as the separation of the beam spots is gradually changed, or atdiscrete separation intervals. It is also proposed to increaseinformation by varying the modulation frequency of the pump beam, and toobtain independent reflectivity measurements at a plurality ofwavelengths using a polychromatic light source.

[0012] U.S. Pat. No. 5,748,317 discloses the use of laser time-delayedpump and probe beams for determining the thermal properties of thinfilm. Measurements of reflectance and other optical characteristics areused to estimate the Kapitza resistance of a film. Inferences regardingthe structure of the film or interfaces therein are made using referencedata obtained from simulation or from another sample. The techniquerequires quantitative conclusions to be drawn about a sample. This isrelatively time-consuming and complex, and is not ideal for the rapidqualitative evaluation of production line output, which large wafers orsimilar specimens may need to be quickly evaluated.

[0013] It is proposed in U.S. Pat. No. 6,253,621 to analyze acousticwaves that are generated in a sample under test in response to a pulsedlaser that is directed to a micro-spot on the sample and scanned.Acoustic waves are detected, and an acoustic index of refraction of aportion of the conductive structure is calculated as a function of thewave. The acoustic index of refraction is then spatially mapped over thesample. It is asserted that defects can be detected by intra-samplecomparisons, or by comparison with a device that is known not to bedefective.

[0014] The above-noted conventional techniques require extensiveanalysis of time-dependent signals. They are slow and computationallyexpensive.

SUMMARY OF THE INVENTION

[0015] The invention improves the quality and rapidity of detection ofsubsurface defects in non-homogeneous structures such as multilayeredintegrated circuits.

[0016] The invention detects buried defects in non-homogeneousstructures such as multilayered integrated circuits without recourse tomeasurements requiring high sensitivity and extensive analysis oftime-dependent signals.

[0017] The invention provides a method and system for transientlyheating a local area of a structure to be tested and thereby producing aphysical deformation therein. The surface of the structure is opticallymonitored while the area cools to a baseline temperature by illuminatingthe local area with one or more probe beams from time to time anddetecting returning light from the probe beams. In some embodiments heatdissipation within the structure is correlated with changes in opticalreflectivity over time. In other embodiments, surface deformation of thestructure is correlated with changes in light that is scattered from thesurface. A first-degree exponential decay curve is constructed from dataobtained from light detectors of the scattered or reflected light, andits decay constant determined. A map may be produced by sequentialapplication of the pump and probe beams to many regions of the structureusing a scanning device. The condition of the structure can then beevaluated by comparing the map of regional decay constants with a mapderived from a non-defective structure. According to the invention,there is no need to construct an estimate of the actual thermalproperties of the sample, or to quantify the Kapitza resistance.Instead, using a pump-probe technique, a flag is raised by the simpledetection of a variation from the expected behavior of the pump-probesystem when applied to a specimen. The pump-probe system is adapted topermit very rapid data acquisition per pixel. Throughput is achieved byelimination of the steps required to characterize an anomaly in aspecimen. It is sufficient to merely identify the presence of theanomaly for purposes of production line quality control.

[0018] The invention provides an optical apparatus for evaluating asample, including beam processing optics for dividing a beam of pulsedcoherent light into a pulsed pump beam and between one and three pulsedprobe beams. The pump beam impinges on the sample for transientexcitation thereof, and the probe beam impinges on the sample at a timesubsequent to impingement thereon by the pump beam. The opticalapparatus further includes a light detector disposed in a return path ofthe probe beam, and an analyzer that receives a signal from the detectorthat is responsive to light detected therein. An anomaly in the sampleis indicated by a difference between returning light from the sample andreturning light from a corresponding point in a reference sample. Theanalyzer may compute a time related function of the signal.

[0019] According to one aspect of the optical apparatus, the lightdetector is also disposed in a return path of the pump beam.

[0020] According to one aspect of the optical apparatus, the timerelated function is a first degree exponential decay curve.

[0021] According to another aspect of the optical apparatus, theanalyzer further determines a decay constant of the curve.

[0022] According to a further aspect of the optical apparatus,responsively to the decay constant, the analyzer generates an indicationof a sub-surface defect in the sample.

[0023] Yet another aspect of the optical apparatus includes a scanningmechanism for scanning the pump beam and the probe beam over the sample.

[0024] According to a further aspect of the optical apparatus, pulses ofthe coherent light have a duration between about 100 fsec and 3 psec.

[0025] According to still another aspect of the optical apparatus,pulses of the coherent light have a duration between about 100 fsec and1 nsec.

[0026] According to yet another aspect of the optical apparatus, thepump beam is incident normal to a surface of the sample.

[0027] According to still another aspect of the optical apparatus, thepump beam is incident oblique to a surface of the sample.

[0028] According to yet another aspect of the optical apparatus, thebeam processing optics process the pump beam and the probe beam bypolarization.

[0029] According to still another aspect of the optical apparatus, thebeam processing optics include a polarizing beamsplitter for dividingthe beam into the probe beam and the pump beam, a retroreflector thatreflects the probe beam, and a non-polarizing beam splitter receivingthe probe beam via the retroreflector, and receiving the pump beam, forcombining the probe beam and the pump beam.

[0030] According to an additional aspect of the optical apparatus, thebeam processing optics process the pump beam and the probe beam bywavelength.

[0031] According to one aspect of the optical apparatus, the beamprocessing optics include a retroreflector that reflects the probe beamand a dichroic mirror for dividing the beam into the probe beam and thepump beam, and for recombining the probe beam and the pump beam.

[0032] According to another aspect of the optical apparatus, the beamprocessing optics also include a second harmonic generator crystaldisposed in an optical path of the beam.

[0033] According to a further aspect of the optical apparatus, the probebeam includes a plurality of temporally dispersed beamlets, and the beamprocessing optics include a plurality of reflective edge filtersdisposed in a path of the beam, and a plurality of retroreflectors, eachretroreflector reflecting one of the beamlets, and a dichroic mirrorthat receives the pump beam and the beamlets via the retroreflectors,and combines the beamlets with the pump beam.

[0034] According to an additional aspect of the optical apparatus, thebeam processing optics include a pair of diffractive gratings disposedin a path of the beam for imposing a frequency chirp on pulses thereof.

[0035] According to one aspect of the optical apparatus, the probe beamincludes a plurality of temporally dispersed beamlets, and the beamprocessing optics includes a plurality of reflective edge filtersdisposed in a path of the beam, and a plurality of retroreflectors, eachof the retroreflectors reflecting one of the beamlets, and furtherincludes focusing optics disposed in paths of the beamlets, wherein thebeamlets impinge on the sample at different angles of incidence.

[0036] According to a further aspect of the optical apparatus, there isa first detector disposed within the specular angular range of the probebeam and a second detector disposed outside the specular angular rangethereof.

[0037] According to another aspect of the optical apparatus, thepolarization of the pump beam differs from the polarization of the probebeam, and there is a first detector disposed in a first return path ofthe pump beam from the sample, and a second detector disposed in asecond return path of the probe beam from the sample, wherein a portionof the first return path avoids the second return path.

[0038] Yet another aspect of the optical apparatus includes a polarizingbeamsplitter disposed in a common segment of the first return path andthe second return path.

[0039] According to still another aspect of the optical apparatus, thepolarizing beamsplitter is disposed within the specular angular range ofthe pump beam and within the specular angular range of the probe beam.

[0040] According to an additional aspect of the optical apparatus, thepolarizing beamsplitter is disposed without the specular angular rangeof the pump beam and without the specular angular range of the probebeam.

[0041] In one aspect of the optical apparatus, there is a first probebeam and a second probe beam, the wavelength of the first probe beamdiffering from that of the second probe beam. The optical apparatusincludes wavelength-responsive collection optics for the first probebeam and the second probe beam, the collection optics projecting thefirst probe beam in a first return path from the sample and projectingthe second probe beam in a second return path from the sample. There isa first detector disposed in the first return path, and a seconddetector disposed in the second return path.

[0042] According to another aspect of the optical apparatus, acollection lens of the collection optics is disposed within the specularangular range of the first probe beam and within the specular angularrange of the second probe beam.

[0043] According to a further aspect of the optical apparatus, acollection lens of the collection optics is disposed without thespecular angular range of the first probe beam and without the specularangular range of the second probe beam.

[0044] In yet another aspect of the optical apparatus, the collectionoptics are disposed within a third return path from the sample of thepump beam, and includes a third detector disposed in the third returnpath.

[0045] According to still another aspect of the optical apparatus, thecollection optics includes a plurality of reflective edge filters.

[0046] According to an additional aspect of the optical apparatus, thecollection optics includes a prism.

[0047] According to one aspect of the optical apparatus, the collectionoptics includes a diffractive grating.

[0048] According to still another aspect of the optical apparatus, theprobe beam includes a first probe beam and a second probe beam, theangle of incidence with the sample of the first probe beam differingfrom the angle of incidence with the sample of the second probe beam.The optical apparatus further includes first collection optics andsecond collection optics that respectively project the first probe beamin a first return path from the sample and the second probe beam in asecond return path from the sample, and wherein the detector includes afirst detector disposed in the first return path and a second detectordisposed in the second return path.

[0049] According to yet another aspect of the optical apparatus, acollection lens of the first collection optics is disposed within thespecular angular range of the first probe beam and a collection lens ofthe second collection optics is within the specular angular range of thesecond probe beam.

[0050] According to a further aspect of the optical apparatus, acollection lens of the first collection optics is disposed without thespecular angular range of the first probe beam and a collection lens ofthe second collection optics is disposed without the specular angularrange of the second probe beam.

[0051] According to another aspect of the optical apparatus, thedetector also includes third collection optics that project the pumpbeam in a third return path from the sample, and includes a thirddetector disposed in the third return path.

[0052] The invention provides a method for evaluating a sample, which iscarried out by impinging a pump beam of pulsed coherent light on thesample for transient excitation thereof, thereafter impinging a pulsedprobe beam on an excited area of the sample, detecting returning lightof the probe beam from the sample, comparing the returning light withcorresponding returning light from a reference sample, and responsivelyto the comparison, indicating the presence of an anomaly in the sample.A time related function of the returning light may be computed.

[0053] Another aspect of the method includes processing the pump beamand the probe beam by polarization.

[0054] A further aspect of the method includes generating a source beamof pulsed coherent light, splitting the source beam to form the pumpbeam and the probe beam, polarizing the pump beam according to a firstpolarization, polarizing the probe beam according to a secondpolarization, projecting the pump beam along a first optical path thatextends to the sample, and projecting the probe beam along a secondoptical path that extends to the sample, wherein the second optical pathis longer than the first optical path.

[0055] Another aspect of the method includes processing the pump beamand the probe beam by wavelength.

[0056] In one aspect of the method the probe beam includes a pluralityof temporally dispersed beamlets, which are impinged on the substrate byprojecting the beamlets along optical paths, each of the paths extendingto the sample, and each of the paths has a different length, wherein thebeamlets impinge on the sample at different angles of incidence.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] For a better understanding of the present invention, reference ismade to the detailed description of the invention, by way of example,which is to be read in conjunction with the following drawings, whereinlike elements are given like reference numerals, and wherein:

[0058]FIG. 1 is a composite illustration of the responses of a defectiveand a non-defective semiconductor structure following transientirradiation according to the present invention;

[0059]FIG. 2 is a high level schematic illustration of an opticalinspection system that is constructed and operative in accordance with adisclosed embodiment of the invention;

[0060]FIG. 3 is a schematic diagram of a photodetector circuit suitablefor use in detectors of the optical inspection system shown in FIG. 2 inaccordance with a disclosed embodiment of the invention;

[0061]FIG. 4 is a schematic illustration of an optical inspection systemthat is constructed and operative in accordance with an alternateembodiment of the invention, in which a pump beam and a probe beamdiffer in polarization;

[0062]FIG. 5 is a schematic illustration of an optical inspection systemthat is constructed and operative in accordance with an alternateembodiment of the invention, in which a probe beam is harmonicallyseparated from a pump beam in frequency;

[0063]FIG. 6 is a schematic illustration of an optical inspection systemthat is constructed and operative in accordance with an alternateembodiment of the invention, in which a probe beam and a pump beam aredivided according to wavelength using a dichroic mirror, and temporallydispersed using a retroreflector;

[0064]FIG. 7 is a schematic illustration of an optical inspection systemthat is constructed and operative in accordance with an alternateembodiment of the invention, in which pump and probe beams are processedtemporally and by wavelength using a series of reflective edge filtersand retroreflectors;

[0065]FIG. 8 is a schematic illustration of an optical inspection systemthat is constructed and operative in accordance with an alternateembodiment of the invention, in which pump and probe beams are processedtemporally and by wavelength using a diffractive grating pair;

[0066]FIG. 9 is a composite plot illustrating temporal dispersion of thepulse generated in the system shown in FIG. 8;

[0067]FIG. 10 is a schematic illustration of an optical inspectionsystem that is constructed and operative in accordance with an alternateembodiment of the invention, in which a plurality of temporallydispersed probe beams impinge a substrate at different angles;

[0068]FIG. 11 is a schematic illustration of a detection subsystem thatdetects reflected light, for use in embodiments of the invention inwhich pump and probe beams are processed according to polarization;

[0069]FIG. 12 is a schematic illustration of a detection subsystem thatdetects scattered light, for use in embodiments of the invention inwhich pump and probe beams are processed according to polarization;

[0070]FIG. 13 is a schematic illustration of a detection subsystememploying a series of edge filters that is constructed and operative foruse in embodiments of the invention in which a plurality of probe beamsdiffer in wavelength;

[0071]FIG. 14 is a schematic illustration of an alternate detectionsubsystem employing a prism that is constructed and operative for use inembodiments of the invention in which a plurality of probe beams differin wavelength;

[0072]FIG. 15 is a schematic illustration of a detection subsystem thatis constructed and operative for use in embodiments of the invention inwhich a plurality of probe beams impinge a substrate at different anglesof incidence; and

[0073]FIG. 16 is a flow chart illustrating a method of detecting burieddefects in opaque films in accordance with a disclosed embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0074] In the following description, numerous specific details are setforth in order to provide a thorough understanding of the presentinvention. It will be apparent to one skilled in the art, however, thatthe present invention may be practiced without these specific details.In other instances well-known circuits, and control logic have not beenshown in detail in order not to unnecessarily obscure the presentinvention.

[0075] Overview.

[0076] By way of introduction, the inventors have discovered that heatdissipation in a non-homogeneous structure, such as an integratedcircuit, is an extremely useful physical effect, which can be exploitedin a particular way in order to detect buried defects. Because heatdissipation is a smooth function of time, only a very small number oftimed measurements are required to analyze a signal. Indeed, in someapplications of the present invention, an adequate analysis can berapidly achieved using as few as 1-3 measurements. The initial form ofthe heating process is not important, as long as it occurs within a timeinterval that is short relative to the period of heat dissipation.

[0077] It has also been found that the transient response to heating isnot very sensitive to initial conditions. Consequently, a single shortpulse of a pump beamlet can be delivered with sufficient intensity forheat dissipation measurements, without recourse to averaging severalpulses.

[0078] Surface heating induces both a reflectivity change and a surfacedeformation in a semiconductor substrate. In embodiments of the presentinvention, these two effects may be measured either simultaneously orindividually in order to track the heat dissipation process. Measurementof specular reflection of a probe beamlet is especially useful fortracking reflectivity changes, and measurement of a deflected probebeamlet is especially useful for tracking surface deformation.

[0079] Heat dissipation causes the temperature of the sample to decay inan exponential fashion, which can be approximated by the equation

T(t)=T₀ e ^(−at),  (1)

[0080] wherein T is temperature, T₀ is initial temperature, t is time,and a is a decay constant. The presence of various defects, such asvoids and interface defects, generally slows the dissipation process,decreasing the value of the decay constant a. Assuming that thereflectivity or surface deformation is proportional to the temperature,the decay constant a may be estimated by measuring the reflected orscattered intensity of a probe beamlet at two or three points in time,and fitting a first degree exponential decay curve to the measurementdata. The inventors have discovered that this estimate is practical, andthat higher order effects may be ignored for purposes of quality controlin a production environment.

[0081] Alternatively, it is practical to estimate the thermal decayusing a simplified formula $\begin{matrix}{{\alpha = \frac{\ln \quad \left( {T_{2} - T_{1}} \right)}{t_{t} - t_{1}}},} & (2)\end{matrix}$

[0082] when there are only 2 or 3 measurements, for example one pumpbeamlet and only 1 or 2 probe beamlets.

[0083] Reference is now made to FIG. 1, which is a compositeillustration of the responses to a pump beamlet of a defective and anon-defective semiconductor structure, each shown in cross-section. Atop row 10 illustrates a thermal effect in response to an incident pumppulse 12 delivered at time t₀ upon a non-defective structure 14, whichis shown at three different times. A bottom row 16 shows correspondingeffects on a structure 18 having a subsurface void 20 that has beensubjected to an identical pump pulse 22 delivered at time to. Ellipses24, 26, 28 represent a front of heat energy that is dissipating withinthe structure 14. Ellipses 30, 32, 34 represent a corresponding front ofheat energy in the structure 18. Generally, as the fronts progressthrough the structures 14, 18, the ellipses increase in area, and theheat energy that was imparted is distributed over an increasingly largerportion of the structures. Thus, for example in the structure 14, theellipse 28 is larger in area than the ellipse 26. Consequently theaverage temperature of that portion of the structure 14 enclosed by theellipse 28 is less than the average temperature of that portion of thestructure 14 enclosed by the ellipse 26. Similarly, the ellipse 32 islarger than the ellipse 30, and the average temperature of that portionof the structure 18 enclosed by the ellipse 32 is less than the averagetemperature of that portion of the structure 18 enclosed by the ellipse30.

[0084] Comparing corresponding ellipses in the top row 10 and the bottomrow 16, initially there is no difference between the ellipse 24 and theellipse 30 at time to, and there is only a small difference between theellipse 26 and the ellipse 32 at time t₁. At time t₂, the ellipse 34 hasa smaller area than the ellipse 28, and the local temperature of theportion of the structure 18 enclosed by the ellipse 34 is higher thanthe area of the structure 14 that is enclosed by the ellipse 28. Thiscan be seen graphically in a plot 36, in which surface temperatureT_(SURF) is plotted against time, wherein a curve 38 and a curve 40correspond to the structure 14 and the structure 18 respectively. Thecurves 38, 40 both decline exponentially according to Equation (1). Itwill be evident that the surface temperature of the defective structure18 declines more slowly than that of the non-defective structure 14. Thedecay constant a (Eq. 1) of curve 40 is smaller than that of curve 38.

[0085] Using an inspection tool according to the present invention in amass production environment, the precise location and character ofdefects need not always be ascertained. For some purposes of qualitycontrol, it is only necessary to flag the presence of a defect. In someembodiments, the presence of a defect may be determined by assigning apredetermined range of allowable values to the decay constant a, outsideof which the sample is considered defective.

[0086] In some embodiments, suitable for patterned semiconductor wafers,values of the decay constant a (Eq. 1) in one sample are compared thoseobtained at corresponding locations of another sample. The values of thedecay constant a (Eq. 1) may be combined with other aspects of opticalsignals described herein to improve throughput and otherwise facilitatedetermination of the presence of a defect, as is disclosed in commonlyassigned application Ser. No. 10/097,442, entitled “Multi-DetectorDefect Detection System and a Method for Detecting Defects”, which isherein incorporated by reference. For instance, the intensity of thereflected pump beamlet can indicate whether or not the beamlet iscurrently incident on a metal surface or on a dielectric surface, thelatter not being of interest in detecting voids. In some embodiments,reflectance information can be fed back to a scanner (not shown), whichcan then advance more rapidly in order in order to reach an area ofgreater interest.

[0087] In some embodiments the inventive techniques disclosed herein arecombined in a more comprehensive system that is capable of inspecting anentire wafer, which includes subsystems that use conventional laserscattering detection arrangements. Such a comprehensive system issimultaneously capable of detecting surface defects, e.g., particles andscratches, as well as buried defects.

[0088] Embodiment 1.

[0089] Turning again to the drawings, reference is now made to FIG. 2,which is a high level schematic illustration of an optical inspectionsystem 42 that is constructed and operative in accordance with adisclosed embodiment of the invention. FIG. 2 illustrates a number offeatures that are common to several other embodiments disclosed infurther detail hereinbelow. The details of such common features aregenerally not repeated in the interest of brevity.

[0090] A modelocked laser source 44 emits a pulsed beam 46 of light,shown representatively as a pulse 48. The laser source 44 can, forexample, be a Vitesse mode-locked Ti:sapphire laser, which produces 100fsec pulses at a wavelength of 800 nm, or a DPM-1000 DPSS mode-lockedNd:YVO laser, which produces 3 psec pulses at a wavelength of 1047 nm.Both of these laser sources are available from Coherent Inc., 5100Patrick Henry Drive, Santa Clara, Calif. 95054 USA. In some embodiments,the laser source 44 may produces pulses having a duration between about100 fsec and 1 nsec. Alternatively, a source of pulsed non-coherentlight can be used to produce the beam 46, except for embodiments havinga DIC configuration.

[0091] The beam 46 enters beam processing optics 47, which includes abeam converter 50, which divides the beam 46 into a pump beamlet 52 andone or more probe beamlets, shown representatively as probe beamlets 54,56. The beam converter 50 also delays the probe beamlets 54, 56 bydifferent time intervals relative to the pump beamlet 52, and mayfurther process the pump beamlet 52 and the probe beamlets 54, 56 so asto enable their discrimination by other characteristics. The beamprocessing optics 47 also include directing optics 58 and focusingoptics 64.

[0092] The split of the beam 46 may be accomplished in some embodimentsof the system 42 by varying the polarization of the beamlets. In suchembodiments, there are only two distinct polarization states, so thatonly one pump beamlet and one probe beamlet are formed, for example thebeamlet 54. This is done such that the polarization of the pump beamlet52 differs from that of the beamlet 54. However, patterned metalinterconnect layers in wafers typically have long, parallel conductors,which respond differently to orthogonal polarizations. When testing suchstructures, polarization dispersion may not be the best method fordiscriminating the pump beamlet 52 and the beamlet 54.

[0093] In other embodiments of the system 42 the pump beamlet 52 and thebeamlets 54, 56 are processed according to wavelength. Short laserpulses from mode-locked lasers comprise a finite spectral range, so thedifferent wavelengths may be selected from within the beam 46 itself.Alternatively, the beam 46 may undergo one or more nonlinear mixingprocesses, e.g., second and third harmonic generation, to produce a setof pulsed probe beamlets 54, 56, which are both completely synchronizedwith the pump beamlet 52, while the pump beamlet 52 and the probebeamlets 54, 56 are distinguished from one another in wavelength.

[0094] In yet other embodiments of the system 42, the directing optics58 cause each of the pump beamlet 52 and the probe beamlets 54, 56 toimpinge on a substrate 60 at a different angle. The probe beamlets 54,56 are then discriminated using a plurality of detectors, eachappropriately situated to collect the specular reflection of only onebeamlet. This technique is suitable for detection of scattered lightfrom surface deformations only if the deviation of the scattered lightfrom the specular reflection angle is small.

[0095] In all embodiments of the system 42, the pump beamlet 52 and theprobe beamlets 54, 56 are dispersed temporally, so that pulses thereineach occupy a unique time subinterval of the interval between successivepulses emitted by the laser source 44. Following application of the pumpbeamlet 52, each of the probe beamlets 54, 56 is incident upon thesubstrate 60 at a different time. Exploitation of reflectance andscattering of the individual beamlets at the different times providesinformation about thermal decay in the substrate 60. The maximum timedelay imparted to any of the probe beamlets 54, 56 must be less than theinterval T between successive pulses of the beam 46 (and the pumpbeamlet 52), in order to maintain a causal relationship between each setof pump and probe pulses. The optimum interval between pulses of thebeam 46 and the time delays of the probe beamlets 54, 56 are allapplication dependent, based on the composition of a particularsubstrate 60 under test. It is desirable that the interval betweenpulses of the beam 46 be at least 10 half lives of thermal decay, inorder to assure that each succeeding pump beamlet arrives at a nearlyfully relaxed surface. Furthermore, a scanner 62, used for scanning thebeamlets over the substrate 60, is typically adjusted such thatsuccessive pump beamlets have some spatial overlap on the substrate 60.It is recommended that the overlap be substantial, or even complete. Inany case, the area covered by the probe beamlets 54, 56 should notextend beyond the area covered by the pump beamlet 52. The time delaysamong the pump beamlet 52 and the probe beamlets 54, 56 are generallyproduced by sending each beamlet along an optical path of differentlength prior to directing it onto the wafer. Some well-known methods ofoptical delay are free-space delay paths, fiber-transmission pathdelays, and chirped pulse stretchers that employ a pair of diffractiongratings or prisms to impart a different delay time to differentspectral components.

[0096] The pump beamlet 52 and the probe beamlets 54, 56 are displacedrelative to the substrate 60 by the scanner 62 in order to inspect agiven area of the substrate 60, and are impinged on the substrate 60 bydirecting optics 58 and focusing optics 64. The pump beamlet 52 and theprobe beamlets 54, 56 can be impinged normal or oblique to the surfaceof the substrate 60.

[0097] The scanner 62, which can be any conventional optical deflectionsystem, e.g., an oscillating mirror, rotating polygon mirror, oracousto-optic deflector, moves the pump beamlet 52 and the probebeamlets 54, 56 across the substrate 60 in a primary scanning direction.In some embodiments, the scanner 62 can be a 2-dimensional scanner, inwhich case the mechanical stage can be omitted. Additionally oralternatively, using conventional scanning techniques, many combinationsof relative rotational and translational motion between the substrate 60and the pump beamlet 52 and the probe beamlets 54, 56 may be employed inorder to optimally scan different regions of the substrate 60. Forexample, the scanner 62 may be programmed to entirely skip regions ofthe substrate 60 in which defects can be well tolerated, or to sparselysample different regions of the substrate 60 that are not too critical,while more essential areas are scanned exhaustively or evenrepetitively. Movement in a secondary scanning direction, whichtypically is orthogonal to the primary scanning direction, is typicallyachieved by mechanical displacement of the substrate 60 relative to thefocusing optics 64. This can be accomplished by a suitable mechanicalstage (not shown).

[0098] Light within the specular angular range of the pump beamlet 52and the probe beamlets 54, 56 returns from the substrate 60 tocollection optics 66 and is detected and processed by a detector 68 anda data sampler 70. Optionally, an additional detector 72 and a datasampler 74 may be provided for light that is scattered by the substrate60.

[0099] The outputs of the data samplers 70, 74 are linked to a suitableanalyzer 76, which approximates exponential curves to the detectioninformation of the detectors 68, 72. The analyzer 76 may have multipleinput channels, and can include output devices, such as displays orplotters (not shown) for displaying the curves and mapping local valuesof the decay constant a (Eq. 1) over the surface of the substrate 60.Such analyzers are well known in the art. For example, the data analysishardware and software of the Compass™ wafer inspection system, availablefrom Applied Materials, Inc. 3050 Bowers Avenue, Santa Clara, Calif.95054, is suitable for the analyzer 76. It will be understood that inthe detection schemes disclosed hereinbelow, in operation, the variousdetectors shown are coupled to the analyzer 76, typically via datasamplers, although the details are generally omitted for clarity.

[0100] The profiles of the spot on the substrate 60 irradiated by thepump beamlet 52 and of the spots illuminated by the probe beamlets 54,56 can have significant effect on the measurement. As has been disclosedwith reference to FIG. 1, thermal diffusion following localizedirradiation occurs both vertically and laterally. Buried voids andinterfaces primarily affect vertical diffusion. Lateral diffusion of theheat actually reduces the effectiveness of the measurements made usingthe detector 68 and the detector 72. Since diffusion depends on thetemperature gradient within the substrate 60, lateral diffusion can beminimized by configuring the pump beamlet 52 to irradiate a pump spothaving a low lateral intensity gradient, e.g., a flat-top profile, inwhich a region containing at least 80% of the incident energy is uniformto within 5%. Lateral dissipation then occurs primarily at the edges ofthe pump spot. Thus, when attempting to detect subsurface voids, thefocusing optics 64 are configured such that the spots illuminated by theprobe beamlets 54, 56 be confined to a probe region that is smaller thanthe pump spot and centered therein. Vertical dissipation is the dominanteffect seen in such a probe region. On the other hand, when attemptingto detect surface deformations using scattered light from the probebeamlets 54, 56, then a flat-top profile of pump beamlet 52 may not beoptimal, as a lateral thermal gradient is desirable in order to producethe required deformation.

[0101] In some embodiments of the system 42 surface deformation of thesubstrate 60 may also be detected using polarized pump-probe beamlets.The pump beamlet 52 and the probe beamlets 54, 56 are arranged in adifferential interference contrast (DIC) configuration, in which casethe laser source 44 must produce coherent light. The intensity and delayof the returning light pulses from the pump and probe beamlets (or twoprobe beamlets) of the same wavelength must be initially adjusted sothat the two beamlets are approximately equal in amplitude, and overlapin time to intentionally create destructive interference when thebeamlets impinge on a defect-free region of the substrate 60. Thepolarizations of the beamlets should also be rotated so that they matchone another. The time delays are adjusted so that the beamlets producenominal destructive interference, in order to provide a zero-backgroundsignal. The detector signals corresponding to each of the probe beamletsthen constitute an interferometric measurement of the displacement ofthe surface substrate 60 relative to its position at the initial timet₀, when the pump beamlet was incident. As thermal relaxation occursover time, the relative surface displacement approaches zeroexponentially.

[0102] It is desirable to provide a balanced photodetector arrangementfor the detectors in the system 42, in order to reduce a large DC offsetand improve the SNR. Reference is now made to FIG. 3, which is aschematic diagram of a photodetector circuit 78 suitable for use in thedetectors 68, 72 in accordance with a disclosed embodiment of theinvention. A photodetector 80 receives a reflected pump beamlet 82, anda photodetector 84 receives a reflected probe beamlet 86. A variableattenuator 88 is placed in the path of one of the pump beamlet 82 andthe probe beamlet 86 in order to equalize the signal outputs of thephotodetectors 80, 84. In general, the pump beamlet 82 and the probebeamlet 86 are separated in time by a delay t_(delay), which is shortrelative to the detector bandwidth, such that the detector isinsensitive to this time delay. The outputs of the photodetectors 80, 84are fed into a differential amplifier 90, which produces an outputsignal on line 92 that represents the difference of the outputs of thephotodetectors 80, 84. To calibrate the circuit 78, the delay t_(delay)is set to 0, and the variable attenuator 88 is adjusted so as to nullthe output of the differential amplifier 90. A model 1807 balancedphotoreceiver, available from New Focus, Inc., 2584 Junction Avenue. SanJose, Calif. 95134, can be used in the detectors 68, 72 to accomplishthe objectives of the circuit 78.

[0103] Embodiment 2.

[0104] Reference is now made to FIG. 4, which is a schematicillustration of an optical inspection system 94 that is constructed andoperative in accordance with an alternate embodiment of the invention. Amodelocked laser source 96 emits a variably polarized output beam 98having a desired ratio of output intensities, as explained hereinbelow.Alternatively, a source of pulsed non-coherent light can be used toproduce the beam 98, except for embodiments having a DIC configuration.The beam 98 is split by a polarizing beamsplitter 100 into a pumpbeamlet 102 and a probe beamlet 104. The ratio of the intensity of thepump beamlet 102 to that of the probe beamlet 104 can be adjusted byadjusting the polarization of the beam 98. The pump beamlet 102 enters anon-polarizing beamsplitter 106, passes through the focusing optics 64,and impinges on the substrate 60. The pump beamlet 102 can impingenormally or obliquely on the substrate 60. The probe beamlet 104 passesthrough a retroreflector 108, and is reflected towards the beamsplitter106 by a reflector 110. The retroreflector 108 is adjustable to providea path varying in length, and thus a variable time delay, for the probebeamlet 104 relative to the pump beamlet 102. The pump beamlet 102 andthe probe beamlet 104 are realigned in the beamsplitter 106 to follow asubstantially common path 112, which passes through the focusing optics64, and impinges on the substrate 60. The common path 112 can impingenormally or obliquely on the surface of the substrate 60, and the pumpbeamlet 102 may be offset from the probe beamlet 104.

[0105] In the system 94, the beamsplitter 100 and the retroreflector 108constitute a beam converter, processing the beam 98 both by polarizationand by time delay. The detection scheme for the system 94 can be any ofthe detection schemes disclosed hereinbelow.

[0106] Embodiment 3.

[0107] Reference is now made to FIG. 5, which is a schematicillustration of an optical inspection system 114 that is constructed andoperative in accordance with an alternate embodiment of the invention.Many details of the system 114 that are identical in the system 94 (FIG.4) are not repeated in the interest of brevity. In the system 114, thepulse 48 undergoes harmonic generation, to produce a set of probepulses, which are completely synchronized with the pump pulse anddistinguished from it in wavelength. The beam 46 passes through a secondharmonic generator (SHG) crystal 116. An emerging beam 118 thus has twodistinct, harmonically related frequency components. The beam 118 issplit by a dichroic mirror 120 into a pump beamlet 122 and a probebeamlet 124, the probe beamlet 124 having a frequency, which is thesecond harmonic of the pump beamlet 122. Alternatively, the pump beamlet122 may be the second harmonic of the probe beamlet 124. The probebeamlet 124 is redirected by the retroreflector 108 and the reflector110. The pump beamlet 122 enters another dichroic mirror 126, where itis realigned with the probe beamlet 124 to follow a substantially commonpath 128, which passes through the focusing optics 64, and impinges onthe substrate 60. The common path 128 can be normal or oblique to thesurface of the substrate 60, and the pump and probe components may beoffset from one another.

[0108] In the system 114, the crystal 116, dichroic mirror 120, andretroreflector 108 constitute a beam converter, processing the pumpbeamlet 122 and the probe beamlet 124 both by wavelength and by timedelay. The detection scheme for the system 114 can be any of thedetection schemes disclosed herein.

[0109] In some embodiments, the crystal 116 can be realized by a crystalfor generating third harmonics. Additionally or alternatively, thecrystal 116 can be realized by a plurality of harmonic generationcrystals, each in its own optical path, so that a plurality ofwavelength-dispersed probe beamlets can be produced.

[0110] Embodiment 4.

[0111] Reference is now made to FIG. 6, which is a schematicillustration of an optical inspection system 130 that is constructed andoperative in accordance with an alternate embodiment of the invention.The system 130 is similar to the system 114 (FIG. 5), except that thecrystal 116 is omitted.

[0112] The system 130 exploits the fact that the ultrafast pulse 48inherently contains a large spectral bandwidth, with a minimum bandwidthΔω given by: Δω·τ≅1. The beam 46 is split by the dichroic mirror 120into a pump beamlet 132 and a probe beamlet 134, the pump beamlet 132having different frequency components from the probe beamlet 134. Theprobe beamlet 134 is redirected by the retroreflector 108 and thereflector 110. The pump beamlet 132 enters another dichroic mirror 126,where it is realigned with the probe beamlet 134 to follow asubstantially common path 136, which passes through the focusing optics64, and impinges on the substrate 60. The common path 136 can impingenormally or obliquely on the surface of the substrate 60, and the pumpbeamlet 132 may be offset from the probe beamlet 134.

[0113] In the system 130, the dichroic mirror 120 and the retroreflector108 constitute a beam converter, processing the pump beamlet 132 and theprobe beamlet 134 both by wavelength and by time delay. The detectionscheme for the system 114 can be any of the detection schemes disclosedherein.

[0114] Embodiment 5.

[0115] Reference is now made to FIG. 7, which is a schematicillustration of an optical inspection system 138 that is constructed andoperative in accordance with an alternate embodiment of the invention.In the system 138 a series of reflective edge filters 140, 142, 144receives the beam 46, and produces multiple beamlets 146, 148, 150, anda pump beamlet 152, each having a unique waveband. The beamlets 146,148, 150 are received respectively by retroreflectors 154, 156, 158. Thebeamlets 146, 148, 150 are then directed to a dichroic mirror 160 byreflectors 162, 164, 166, where they are realigned with the pump beamlet152 to follow a substantially common path 168.

[0116] The retroreflectors 154, 156, 158 form free-space delay lines.They are disposed so that the optical paths of the beamlets 146, 148,150, and the pump beamlet 152 are all of different lengths. Thus, thereflective edge filters 140, 142, 144 and the retroreflectors 154, 156,158 cooperate to constitute both a temporal and wavelength beamconverter for the beam 46.

[0117] The common path 168 passes through focusing optics 64, andimpinges on the substrate 60, each component arriving at a differenttime. The common path 168 can be normal or oblique to the surface of thesubstrate 60, and the pump and probe components may be offset from oneanother. The detection scheme for the system 114 can be any of thedetection schemes disclosed herein.

[0118] Embodiment 6.

[0119] Reference is now made to FIG. 8, which is a schematicillustration of an optical inspection system 170 that is constructed andoperative in accordance with an alternate embodiment of the invention.In the system 170, a parallel diffractive grating pair 172 is introducedinto the path of the beam 46. The grating pair 172 introduces a groupdelay, stretching the pulse 48, to produce a frequency-chirped beam 174.The grating pair 172 is configured by choosing the distance between thepair, the incidence angle of the beam 46, and the grating period in aknown manner, so as to spread the pulse across all or part of the timeinterval between successive pulses emitted by the laser source 44. Thebeam 174 actually consists of a train of pulses 176, all occurringwithin the time interval T.

[0120] In the system 170, the grating pair 172 constitutes a beamconverter, processing the beam 46 both by wavelength and by time delay.Alternatively, a prism pair may be used for this same purpose, as isknown in the art. The detection scheme for the system 170 can be any ofthe detection schemes disclosed herein.

[0121] Reference is now made to FIG. 9, which is a composite plotillustrating temporal dispersion of the pulse 48 (FIG. 8). A peak 178,which represents the first pulse in the train of pulses 176 (FIG. 8), isa pump pulse. Succeeding peaks 180, 182, 184 correspond to probe pulses.Each of the peaks 178, 180, 182, 184 has a different waveband, and isdelayed differently from the others.

[0122] Embodiment 7.

[0123] Reference is now made to FIG. 10, which is a schematicillustration of an optical inspection system 186 that is constructed andoperative in accordance with an alternate embodiment of the invention.In this embodiment, the beam 46 is split into a pump beamlet 188, and aplurality of probe beamlets 190, 192, 194 by beamsplitters 196, 198,200, respectively. The pump beamlet 188 is directed by the directingoptics 58 through a beamsplitter 202 and focusing optics 64, andimpinges on the substrate 60 either normally or obliquely.

[0124] The relative reflectance and transmittance of each of thebeamsplitters 196, 198, 200 are typically (but not necessarily) chosenso that all the beamlets 190, 192, 194 have equal intensities. Thebeamlets 190, 192, 194 are retroreflected respectively byretroreflectors 154, 156, 158. The beamlets 190, 192, 194 are thenredirected by reflectors 204, 206, 208 generally toward the substrate 60through focusing optics 210, 212, 214, respectively. The reflectors 204,206, 208 are arranged so as to impinge the beamlets 190, 192, 194 on thesubstrate 60 at different angles. The spots illuminated by the beamlets190, 192, 194 and the pump beamlet 188 may be coincident, or offset fromone another in many different combinations.

[0125] The retroreflectors 154, 156, 158 form free-space delay lines.They are disposed so that the optical paths of the beamlets 190, 192,194 are of different lengths. Thus, the beamsplitters 196, 198, 200, theretroreflectors 154, 156, 158, and the reflectors 204, 206, 208cooperate to constitute a temporal beam converter for the beam 46. Thedetection scheme for the system 186 can be any of the detection schemesdisclosed herein. An advantage of the embodiment represented by thesystem 186 is that it does not require multiple wavelengths for thebeamlets 190, 192, 194. Thus, a narrow-spectrum laser can be used as thelaser source 44. Furthermore, there is good physical separation betweenthe reflected beamlets, eliminating possible crosstalk in collinearembodiments. In some applications, crosstalk can be even further reducedby using combinations of angular and spectral separation of the beamlets190, 192, 194.

[0126] Embodiment 8.

[0127] Reference is now made to FIG. 11, which is a schematicillustration of a detection subsystem 216 that is constructed andoperative for use in alternate embodiments of the invention. Thesubsystem 216 is particularly useful in embodiments of the invention inwhich the pump and probe beamlets are processed according topolarization, for example the system 94 (FIG. 4). The subsystem 216 isdescribed with reference to FIG. 4 by way of example, it beingunderstood that the disclosure is applicable to other embodimentsherein.

[0128] Beam collecting optics 218 are mounted within the specularangular range of pump and probe beamlets 220, for example the probebeamlet 104 (FIG. 4), to capture a reflected beam 222 from the pump andprobe beamlets that return from the substrate 60. The specular angularrange is represented by an angle 224, The collecting optics 218 may bethe same as or different from the focusing optics 64 (FIG. 4). The beam222 enters a polarizing beamsplitter 226, where it is divided into aprobe beamlet 228 and a pump beamlet 230, which are detectedrespectively by a probe detector 232 and a pump detector 234. As notedabove, the probe detector 232 and pump detector 234 may be fast singleelement detectors or fast imaging detectors. The purpose of collectinginformation from the pump detector 234 regarding the pump beamlet 230 isto establish baseline conditions at time t=0. The pump detector 234 seesthe substrate during the initial rise in temperature, and also providesa reference for the probes, compensating for variations betweendifferent specimens due to surface roughness, line width, preciseposition over the pattern, etc. These physical characteristics can causevariations in the signals, which are unrelated to thermal effects. Theseeffects can be eliminated by dividing probe signals by pump signals toproduced normalized signals. Furthermore, as discussed above, thesignals due to temperature changes are relatively small and are carriedon a larger DC signal. The DC component from the pump beamlet 230 can besubtracted from the probe beamlet 228 using balanced photodetectors asdisclosed above with respect to Embodiment 1. The probe detector 232collects information at different points during the time interval T toestablish the decay constant a (Eq. 1) that applies to the region of thesubstrate 60 currently being illuminated.

[0129] A detector 236 is disposed outside the angle 224 and detectsscattered light from the pump beamlet and the probe beamlets. A fastsingle element detector can be used as the detector 236. Alternatively,the detector 236 can be realized by multiple detectors, disposed atdifferent points surrounding the point of illumination. The detector 236should be as far as possible outside the specular angle. Optimally theangle included by the incident beam and a ray 238 leading from thesubstrate 60 to the detector 236 should be 90 degrees. Smaller anglesare also useful, but the angle between the incident beam and the ray 238should be at least twice as large as the NA of the illumination optics,that is the half-angle of the illumination cone.

[0130] Embodiment 9.

[0131] Reference is now made to FIG. 12, which is a schematicillustration of a detection subsystem 240 that is constructed andoperative for use in alternate embodiments of the invention. Thesubsystem 240 is similar to the subsystem 216 (FIG. 11). However, in thesubsystem 240 the pump and probe beamlets 220 arrive at the substrate 60from a direction that is outside the angle 224. Now the probe detector232 and the pump detector 234 detect scattered rather than reflectedlight. An additional detector 236 may be used to detect scattered lightfrom the pump and/or probe beamlets. The probe detector 232 and the pumpdetector 234 should collect light within the cone of specularlyreflected light but should be de-centered. For example, using anillumination cone of ±10 degrees from the normal direction, thecollection optics should collect an angle of ±2 degrees, centered at 2degrees to one side of the normal angle. The probe beamlet also bedisplaced to one side of the pump beamlet by about half the beamletdiameter, so as to receive a well-defined deflection caused by surfacedistortion. In this embodiment, the pump and probe beamlets do not fullyoverlap.

[0132] Embodiment 10.

[0133] Reference is now made to FIG. 13, which is a schematicillustration of a detection subsystem 242 that is constructed andoperative for use in alternate embodiments of the invention. Thesubsystem 242 is particularly useful in those embodiments in which theprobe beamlets have been processed by wavelength, for example the system114 (FIG. 5).

[0134] The beam 222 that is reflected or scattered from the substrate 60passes through collecting optics 218. Probe beamlets 244, 246, 248,which are components of the beam 222, and which have differentwavebands, are redirected by a reflector 250 through a series ofreflective edge filters 252, 254, 256 to corresponding probe detectors258, 260, 262. The edge filters 252, 254, 256 are each configured toredirect one of the probe beamlets 244, 246, 248, and to allow the otherprobe beamlets to continue onward. A pump beamlet 264, which is anothercomponent of the beam 222, passes through all of the edge filters 252,254, 256, and continues toward a pump detector 266. Because the probebeamlet components of the beam 222 are pre-sorted by the edge filters252, 254, 256, the detectors 258, 260, 262 are not required to bewaveband selective.

[0135] In the subsystem 242 the collecting optics 218 may be locatedwithin or without the specular angular range of the incident pump andprobe beamlets (not shown). In the latter case, the detector 236 may beomitted if desired, or it may be used to collect specularly reflectedradiation, as the detectors 258, 260, 262, 266 already function asdark-field detectors.

[0136] Embodiment 11.

[0137] Reference is now made to FIG. 14, which is a schematicillustration of a detection subsystem 268 that is constructed andoperative for use in alternate embodiments of the invention. Thesubsystem 268 is similar to the subsystem 242 (FIG. 13), and provideswavelength dispersion of the beam 222. However in the subsystem 268,this is achieved using a prism 270, in place of the edge filters 252,254, 256 (FIG. 13). Other dispersive elements, as are known in the art,such as a grating, may be used in place of the prism 270.

[0138] In the subsystem 268, the collecting optics 218 may be locatedwithin or without the specular angular range of the incident pump andprobe beamlets (not shown). In the latter case, the detector 236 may beused to collect specularly reflected radiation, as the detectors 258,260, 262, 266 already function as dark-field detectors.

[0139] Embodiment 12.

[0140] Reference is now made to FIG. 15, which is a schematicillustration of a detection subsystem 272 that is constructed andoperative for use in alternate embodiments of the invention. In thesubsystem 272, a pump beamlet 274 reaches the substrate 60 via focusingoptics 64. The pump beamlet 274 can be generated using any of theembodiments disclosed herein, and can impinge normally or obliquely onthe surface of the substrate 60. A plurality of probe beamlets 276, 278,280 are directed to the substrate 60 by respective focusing optics 282,284, 286. The probe beamlets 276, 278, 280 may be generated by any ofthe embodiments disclosed herein that result in spatial separation of aplurality of probe beamlets. For example, the system 186 (FIG. 10) wouldbe suitable. Alternatively, the probe beamlets 276, 278, 280 may begenerated using any other method of generating a plurality of pulsedbeams. It should be noted that the subsystem 272 is not suitable for DICconfiguration.

[0141] The spots on the substrate 60 that are illuminated by the probebeamlets 276, 278, 280 may coincide with the spot illuminated by thesubsystem 272, or be offset from one another in many differentcombinations. In any case, the angles of incidence of the probe beamlets276, 278, 280 with the substrate 60 are different. The angles ofreflection of the probe beamlets 276, 278, 280 are thereforecorrespondingly different at a given point on the detectors 258, 260,262.

[0142] Reflected light from the probe beamlets 276, 278, 280 passesrespectively through collecting optics 288, 290 292. The reflection ofthe pump beamlet 274 returns via the focusing optics 64. In someembodiments separate collection optics (not shown) could be included inthe path of the reflection of the pump beamlet 274. Each of thecollecting optics 288, 290 292 are located outside the specular angularrange of all the probe beamlets 276, 278, 280 other than its respectiveprobe beamlet.

[0143] The reflection of the pump beamlet 274 is detected by a detector294. The reflections of the probe beamlets 276, 278, 280 are detected byappropriately positioned detectors 296, 298, 300, respectively. Inembodiments where the detectors 296, 298, 300 function as dark-fielddetectors, they should be photomultiplier tubes. When the detectors 296,298, 300 detect specular reflections, they may be photomultiplier tubes,but preferably are PIN diodes in order to achieve a high dynamic range,due to the relatively large signal from the reflected beamlets.

[0144] In the above disclosure the collecting optics 288, 290 292 arepositioned within the specular angular range of their respective probebeamlets 276, 278, 280 in order to evaluate reflected light.Alternatively, the collecting optics 288, 290, 292 can be positionedoutside the specular angular range of all the probe beamlets 276, 278,280, in which case the detectors 296, 298, 300 function as dark-fielddetectors. In such embodiments, separate collection optics (not shown)may be provided for the detector 294, or the detector 294 may even beomitted.

[0145] In some embodiments the probe beamlets 276, 278, 280 may betemporally distributed, as disclosed in various embodiments herein, forexample the system 170 (FIG. 8).

[0146] In other embodiments of the subsystem 272, the probe beamlets276, 278, 280 may simultaneously arrive at the substrate 60 at the sameor different spots, which simplifies the beam processing optics.

[0147] Operation.

[0148] Reference is now made to FIG. 16, which is a flow chartillustrating a method of detecting buried defects in opaque films inaccordance with a disclosed embodiment of the invention. The processbegins at initial step 302, in which an apparatus constructed accordingone of the embodiments described above is configured.

[0149] Next, at step 304, a reference specimen is mounted on theapparatus.

[0150] Next, at step 306, the specimen is scanned to a point ofinterest. In some applications all points of the specimen may be scannedat a predetermined resolution, while in other applications, onlyselected points of interest are evaluated.

[0151] Next, at step 308, data is collected from the point at which thescanner was positioned at step 306. The specimen is irradiated by a pumpbeamlet as described herein, and then by a succession of probe beamlets,according to any of the embodiments disclosed hereinabove. In someapplications only one probe beamlet suffices. No more than three probebeamlets are necessary in any case, according to the invention. The rawdata taken from the photodetectors may be stored. Alternatively, the rawdata may be normalized and stored for ease of comparison with testspecimens. Alternatively, a decay constant may be computed in accordancewith Equation (1) or Equation (2) and stored. Whichever approach isselected, data from test specimens is treated identically, as explainedhereinbelow.

[0152] Control now proceeds to decision step 310, where a determinationis made whether more points of the reference specimen remain to beevaluated. If the determination at decision step 310 is affirmative,then control returns to step 306.

[0153] If the determination at decision step 310 is negative, thencontrol proceeds to step 312, where a test specimen is mounted on theapparatus in the same manner as the reference specimen in step 304.

[0154] Next, at step 314, the scanner is positioned. The same scanningoperations are executed in step 314 as were performed in step 306, so asto evaluate corresponding points of the test and reference specimens.

[0155] Next, at step 316, data is collected from the current point ofthe test specimen in the same manner as was done in step 308, thedetails of which are not repeated.

[0156] Control now proceeds to decision step 318, where a comparison ofthe data obtained in step 316 is made with data from the correspondingpoint of the reference specimen. The statistical methods taught in theabove noted application Ser. No. 10/097,442 may be used for thecomparison. Alternatively data reduction methods employing Equation (2)may be used. In any case, the result is compared with a predeterminedtolerance or standard, which is application specific, and it isdetermined if the measurement at the current point exceeds thepredetermined tolerance.

[0157] If the determination at decision step 318 is affirmative, thencontrol proceeds to step 324, which is disclosed hereinbelow.

[0158] If the determination at decision step 318 is negative, thencontrol proceeds to decision step 320, where a determination is madewhether more points of the test specimen remain to be evaluated.

[0159] If the determination at decision step 320 is affirmative, thencontrol returns to step 314.

[0160] If the determination at decision step 320 is negative, thencontrol proceeds to final step 322, and the procedure terminates.Subsequent test specimens may be evaluated by entering the flow chart atstep 312, as the reference data need not be generally recollected,provided that manufacturing conditions are unchanged.

[0161] Step 324 is performed if the determination at decision step 318is affirmative. It is concluded that an anomaly is present. No furthersteps need be taken to characterize the anomaly. In a production lineenvironment, it may be required to immediately reject the test specimenas being of substandard quality. Accordingly, control proceeds to finalstep 322, as there is no need to delay production by continuing thescan. Alternatively, it may be desirable to identify all anomalies ofthe test specimen in order to adjust the manufacturing process moreefficiently, or for other purposes of evaluation. In such applicationscontrol proceeds to decision step 320, as indicated by a dotted line inthe flow chart of FIG. 16.

[0162] It will be appreciated by persons skilled in the art that thepresent invention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

1. An optical apparatus for evaluating a sample, comprising: beamprocessing optics for dividing a beam of pulsed light into a pumpbeamlet and a probe beamlet, said pump beamlet impinging on said samplefor transient excitation thereof, said probe beamlet impinging on saidsample subsequent to impingement thereon by said pump beamlet; a lightdetector disposed in a return path of said probe beamlet; and ananalyzer, receiving a signal from said detector that is responsive tolight detected therein for comparing said signal with a correspondingsignal obtained from a reference sample, wherein a presence of ananomaly in said sample is indicated by a deviation of said signal fromsaid corresponding signal.
 2. The optical apparatus according to claim1, wherein said probe beamlet is exactly three probe beamlets thatimpinge on said sample at different times.
 3. The optical apparatusaccording to claim 1, wherein said probe beamlet is exactly two probebeamlets that impinge on said sample at different times.
 4. The opticalapparatus according to claim 1, wherein said analyzer is adapted tocompare a first degree exponential decay of said signal with a firstdegree exponential decay of said corresponding signal.
 5. The opticalapparatus according to claim 1, further comprising a scanning mechanism,for scanning said pump beamlet and said probe beamlet over said sample.6. The optical apparatus according to claim 1, wherein said lightdetector is disposed in a return path of said pump beamlet.
 7. Theoptical apparatus according to claim 1, wherein pulses of said lighthave a duration between about 100 fsec and 3 psec.
 8. The opticalapparatus according to claim 1, wherein pulses of said light have aduration between about 100 fsec and 1 nsec.
 9. The optical apparatusaccording to claim 1, wherein said pump beamlet is incident normal to asurface of said sample.
 10. The optical apparatus according to claim 1,wherein said pump beamlet is incident oblique to a surface of saidsample.
 11. The optical apparatus according to claim 1, wherein saidbeam processing optics process said pump beamlet and said probe beamletby polarization.
 12. The optical apparatus according to claim 1, whereinsaid beam processing optics comprise: a polarizing beamsplitter fordividing said beam into said probe beamlet and said pump beamlet; aretroreflector for reflecting said probe beamlet; and a non-polarizingbeam splitter receiving said probe beamlet via said retroreflector, andreceiving said pump beamlet, for directing said probe beamlet and saidpump beamlet along a substantially common optical path.
 13. The opticalapparatus according to claim 1, wherein said beam processing opticsprocess said pump beamlet and said probe beamlet by wavelength.
 14. Theoptical apparatus according to claim 1, wherein said beam processingoptics comprise a retroreflector, said retroreflector reflecting saidprobe beamlet and a dichroic mirror for dividing said beam into saidprobe beamlet and said pump beamlet, and for redirecting said probebeamlet and said pump beamlet along a substantially common optical path.15. The optical apparatus according to claim 14, wherein said beamprocessing optics further comprises a second harmonic generator crystaldisposed in an optical path of said beam.
 16. The optical apparatusaccording to claim 1, wherein said probe beamlet comprises a pluralityof temporally dispersed probe beamlets, and said beam processing opticscomprises: a plurality of reflective edge filters disposed in a path ofsaid beam; a plurality of retroreflectors, each of said retroreflectorsreflecting one of said probe beamlets; and a dichroic mirror, receivingsaid pump beamlet and receiving said probe beamlets via saidretroreflectors, for combining said probe beamlets with said pumpbeamlet.
 17. The optical apparatus according to claim 1, wherein saidbeam processing optics comprises a pair of diffractive gratings disposedin a path of said beam for imposing a frequency chirp on pulses thereof.18. The optical apparatus according to claim 1, wherein said probebeamlets comprise a plurality of temporally dispersed beamlets, and saidbeam processing optics comprises: a plurality of reflective edge filtersdisposed in a path of said beam; a plurality of retroreflectors, each ofsaid retroreflectors reflecting one of said temporally dispersedbeamlets; and focusing optics disposed in paths of said temporallydispersed beamlets, wherein said temporally dispersed beamlets impingeon said sample at different angles of incidence.
 19. The opticalapparatus according to claim 1, wherein said detector comprises a firstdetector disposed within a specular angular range of said probe beamletand a second detector disposed outside said specular angular rangethereof.
 20. The optical apparatus according to claim 1, wherein apolarization of said pump beamlet differs from a polarization of saidprobe beamlet, and said detector comprises: a first detector disposed ina first return path of said pump beamlet from said sample; and a seconddetector disposed in a second return path of said probe beamlet fromsaid sample, wherein a portion of said first return path avoids saidsecond return path.
 21. The optical apparatus according to claim 20,further comprising a polarizing beamsplitter disposed in a commonsegment of said first return path and said second return path.
 22. Theoptical apparatus according to claim 21, wherein said polarizingbeamsplitter is disposed within a specular angular range of said pumpbeamlet and within a specular angular range of said probe beamlet. 23.The optical apparatus according to claim 21, wherein said polarizingbeamsplitter is disposed without a specular angular range of said pumpbeamlet and without a specular angular range of said probe beamlet. 24.The optical apparatus according to claim 1, wherein said probe beamletcomprises a first probe beamlet and a second probe beamlet, a wavelengthof said first probe beamlet differing from a wavelength of said secondprobe beamlet, further comprising wavelength-responsive collectionoptics for said first probe beamlet and said second probe beamlet, saidcollection optics projecting said first probe beamlet in a first returnpath from said sample and projecting said second probe beamlet in asecond return path from said sample; and wherein said detector comprisesa first detector disposed in said first return path and a seconddetector disposed in said second return path.
 25. The optical apparatusaccording to claim 24, wherein a collection lens of said collectionoptics is disposed within a specular angular range of said first probebeamlet and within a specular angular range of said second probebeamlet.
 26. The optical apparatus according to claim 24, wherein acollection lens of said collection optics is disposed without a specularangular range of said first probe beamlet and without a specular angularrange of said second probe beamlet.
 27. The optical apparatus accordingto claim 24, wherein said collection optics are disposed within a thirdreturn path from said sample of said pump beamlet, and furthercomprising a third detector disposed in said third return path.
 28. Theoptical apparatus according to claim 24, wherein said collection opticscomprises a plurality of reflective edge filters.
 29. The opticalapparatus according to claim 24, wherein said collection opticscomprises a prism.
 30. The optical apparatus according to claim 24,wherein said collection optics comprises a diffractive grating.
 31. Theoptical apparatus according to claim 1, wherein said probe beamletcomprises a first probe beamlet and a second probe beamlet, an angle ofincidence with said sample of said first probe beamlet differing from anangle of incidence with said sample of said second probe beamlet,further comprising first collection optics and second collection opticsthat respectively project said first probe beamlet in a first returnpath from said sample and said second probe beamlet in a second returnpath from said sample; and wherein said detector comprises a firstdetector disposed in said first return path and a second detectordisposed in said second return path.
 32. The optical apparatus accordingto claim 31, wherein a collection lens of said first collection opticsis disposed within a specular angular range of said first probe beamletand a collection lens of said second collection optics is within aspecular angular range of said second probe beamlet.
 33. The opticalapparatus according to claim 31, wherein a collection lens of said firstcollection optics is disposed without a specular angular range of saidfirst probe beamlet and a collection lens of said second collectionoptics is disposed without a specular angular range of said second probebeamlet.
 34. The optical apparatus according to claim 31, wherein saiddetector further comprises third collection optics that project saidpump beamlet in a third return path from said sample, and furthercomprising a third detector disposed in said third return path.
 35. Amethod for evaluating a sample, comprising the steps of: impinging apump beamlet of pulsed light on an area of said sample for transientexcitation thereof; thereafter impinging a probe beamlet on an excitedarea of said sample; detecting returning light of said probe beamletfrom said sample; impinging another instance of said pump beamlet on areference sample; thereafter impinging another instance of said probebeamlet on an excited area of said reference sample; comparing saidreturning light of said probe beamlet with returning light from saidanother instance of said probe beamlet; and responsively to said step ofcomparing reporting an anomaly of said sample upon detection of adifference between said returning light of said probe beamlet and saidreturning light from said another instance of said probe beamlet. 36.The method according to claim 35, wherein said probe beamlet is exactlytwo probe beamlets.
 37. The method according to claim 35, wherein saidprobe beamlet is exactly three probe beamlets.
 38. The method accordingto claim 35, wherein said step of comparing comprises computing a timerelated function of said returning light of said probe beamlet and saidreturning light from said another instance of said probe beamlet. 39.The method according to claim 38, wherein said time related function isa first degree exponential decay curve.
 40. The method according toclaim 39, further comprising the step of determining a decay constant ofsaid curve.
 41. The method according to claim 35, wherein pulses of saidlight have a duration between about 100 fsec and 3 psec.
 42. The methodaccording to claim 35, wherein pulses of said light have a durationbetween about 100 fsec and 1 nsec.
 43. The method according to claim 35,wherein said pump beamlet is incident normal to a surface of saidsample.
 44. The method according to claim 35, wherein said pump beamletis incident oblique to a surface of said sample.
 45. The methodaccording to claim 35, further comprising the step of processing saidpump beamlet and said probe beamlet by polarization.
 46. The methodaccording to claim 35, wherein said steps of impinging a pump beamletand impinging a probe beamlet are performed by: generating a source beamof pulsed light; splitting said source beam to form said pump beamletand said probe beamlet; polarizing said pump beamlet according to afirst polarization; polarizing said probe beamlet according to a secondpolarization; projecting said pump beamlet along a first optical paththat extends to said sample; and projecting said probe beamlet along asecond optical path that extends to said sample, wherein said secondoptical path is longer than said first optical path.
 47. The methodaccording to claim 35, further comprising the step of processing saidpump beamlet and said probe beamlet by wavelength.
 48. The methodaccording to claim 35, wherein said probe beamlet comprises a pluralityof temporally dispersed probe beamlets, and said step of impinging aprobe beamlet is performed by: projecting said temporally dispersedprobe beamlets along optical paths, each of said paths extending to saidsample, and each of said paths having a different length, wherein saidtemporally dispersed probe beamlets impinge on said sample at differentangles of incidence.
 49. The method according to claim 48, wherein saidstep of detecting returning light further comprises detecting a firstlight of a first one of said temporally dispersed probe beamlets in afirst return path from said sample and detecting a second light of asecond one of said temporally dispersed probe beamlets in a secondreturn path from said sample.
 50. The method according to claim 49,wherein at least a portion of said first return path lies within aspecular angular range of said first one of said temporally dispersedprobe beamlets and at least a portion of said second return path lieswithin a specular angular range of said second one of said temporallydispersed probe beamlets.
 51. The method according to claim 49, whereinat least a portion of said first return path lies without a specularangular range of said first one of said temporally dispersed probebeamlets and at least a portion of said second return path lies withouta specular angular range of said second one of said temporally dispersedprobe beamlets.
 52. The method according to claim 48, wherein said stepof detecting returning light further comprises detecting a third lightof said pump beamlet in a third return path from said sample.
 53. Themethod according to claim 35, wherein a polarization of said pumpbeamlet differs from a polarization of said probe beamlet, and said stepof detecting returning light is performed by: detecting a first light ina first return path of said pump beamlet from said sample; and detectinga second light in a second return path of said probe beamlet from saidsample, wherein a portion of said first return path avoids said secondreturn path.
 54. The method according to claim 53, wherein said firstreturn path and said second return path share a common segment, and saidstep of detecting returning light further comprises processing saidprobe beamlet and said pump beamlet in said common segment according topolarization.
 55. The method according to claim 54, wherein said commonsegment is disposed within a specular angular range of said pump beamletand within a specular angular range of said probe beamlet.
 56. Themethod according to claim 54, wherein said common segment is disposedwithout a specular angular range of said pump beamlet and without aspecular angular range of said probe beamlet.
 57. The method accordingto claim 35, wherein a wavelength of said pump beamlet differs from awavelength of said probe beamlet, and said step of detecting returninglight is performed by: detecting a first light in a first return path ofsaid pump beamlet from said sample; and detecting a second light in asecond return path of said probe beamlet from said sample, wherein aportion of said first return path avoids said second return path. 58.The method according to claim 57, wherein said first return path andsaid second return path share a common segment, and said step ofdetecting returning light further comprises processing said probebeamlet and said pump beamlet in said common segment according towavelength.
 59. The method according to claim 58, wherein said commonsegment is disposed within a specular angular range of said pump beamletand within a specular angular range of said probe beamlet.
 60. Themethod according to claim 58, wherein said common segment is disposedwithout a specular angular range of said pump beamlet and without aspecular angular range of said probe beamlet.